A central goal of neurobiology is to understand how the brain forms, stores, retrieves, modifies and encodes information, and to determine how these operations go awry in neurological and psychiatric diseases. The focus of this application is astrocytes, a type of glia. Long considered simply the brain's glue, astrocytes are emerging as important regulators of neuronal function. Astrocytes are ubiquitous, highly branched cells that tile the entire central nervous system, making contacts with neurons and blood vessels, and serving diverse roles. Established roles include ion homeostasis, neurotransmitter clearance, synapse formation/removal, synaptic modulation and contributions to neurovascular coupling. Deciphering and exploiting the physiological roles of astrocytes in the brain is one of the major open questions in neuroscience. This R35 application seeks to exploit technical and conceptual advances made with R01, R21 and DP1 awards and combine them into a single nimble, long-term research program to systematically explore and comprehensively understand the fundamental biology of astrocytes within adult vertebrate intact neural circuits with the compass-driven goal of exploiting this information for advancing new therapies. In this context, we define dysfunction as astrocyte process withdrawal from synapses or altered astrocyte signaling, including trophic support, to synapses. Such dysfunctions would alter established astrocyte roles including neurotransmitter clearance, synapse regulation and maintenance. This in turn would alter the timing of synaptic transmission and microcircuit function, contribute to excitotoxicity and perhaps trigger synapse removal. By exploiting novel experimental tools and concepts generated as part of DP1 and R21 awards, and by applying them to the exemplar striatal microcircuitry studied as part of successive R01 awards, we will determine how astrocytes regulate intact neural microcircuits in vivo. We will test the overarching hypothesis that astrocytes represent a hitherto largely overlooked mechanism in neural circuit function and in neurological disorders. As part of these efforts, we will utilize, and if necessary develop, state-of-the-art tools for molecular, cellular and circuit levels of evaluation. As described herein, our long-term programmatic goal, therefore, is to deliver pivotal molecular, physiological and mechanistic insights on astrocyte contributions to brain function and disease, laying the groundwork for therapeutic advances to occur. We will continue to share openly our database resources and tools in order to enable additional advances by others. In addition, the research program represents an outstanding opportunity and laboratory environment for training the next generation of scientists and physician-scientists.